In recent fields of technologies such as servers and high-performance computers (HPCs), improvements in performance made by the use of multiple central processing units (CPUs) have dramatically increased the transmission capacity of input/output (I/O) devices that are responsible for communication between the CPUs and external interfaces.
Meanwhile, high-speed transmission using electrical signals is limited from the viewpoints of the occurrence of crosstalk and the wiring density. Hence, studies are being made on a technique that realizes high-speed I/O with photoelectric conversion elements using light signals (optical interconnection).
To realize optical interconnection, an optical transmitter-and-receiver module is to be prepared. Exemplary optical transmitter-and-receiver modules include an optical module in which elements such as a light-emitting element and a light-receiving element are mounted face down on a substrate, and a polymer optical waveguide provided below the substrate is directly connected to the substrate. Polymer optical waveguides realize low-cost optical connection in optical modules including light-transmitting-and-receiving elements.
Polymer optical waveguides, however, have a propagation loss of about 0.04 dB/cm, which is larger than a propagation loss of about 2.4 dB/km occurring in multi-mode optical fibers. Considering that the length of optical wires provided on server boards is about 20 cm, there is a proposal (T. Shiraishi et al., OTuQ5, OFC 2011, for example) in which a polymer optical waveguide is employed for an optical transmitter-and-receiver module provided on a server board while a multi-mode optical fiber is employed for an optical transmitter-and-receiver module provided on a backplane, on which optical wires having lengths of about 1 m are provided.
There are several kinds of polymer optical waveguides manufactured by different methods (“Introduction of optical circuit board”, Sumitomo Bakelite Co., Ltd., http://www.jpca.net/hikari/db/sumibe01.pdf, for example).
FIGS. 1A and 1B illustrate a method of forming a core by exposure and development (a direct exposure method) and a method of forming a cladding by exposure (a photo-addressing method), respectively, as typical examples. In the method of forming a core by exposure and development illustrated in FIG. 1A, a core layer 1002 is provided on a lower cladding layer 1001 by lamination, and a portion of the core layer 1002 that is to become the core is exposed to light through a mask 1005 and is then developed, whereby a core 1003 is obtained.
Subsequently, an upper cladding layer 1004 is provided over the core 1003 by lamination. Lastly, the resultant body is baked. Thus, an optical waveguide is obtained. In this method, if the lower cladding layer 1001 and the upper cladding layer 1004 are made of the same material, the difference in refractive index between the core and the cladding may be easily made substantially the same for a direction (vertical direction) perpendicular to and a direction (horizontal direction) parallel to a surface on which the optical waveguide is held.
Such a characteristic is referred to as “isotropy” of the difference in refractive index. In this method, however, the sidewalls of the core 1003 obtained through development have some surface roughness, leading to some propagation loss that deteriorates optical performance.
In the method of forming a cladding by exposure illustrated in FIG. 1B, a lower cladding layer 1011, a core layer 1012, and an upper cladding layer 1014 are stacked by lamination, and portions of the core layer 1012 excluding a portion that is to become the core is exposed to light, whereby the refractive index of the portions of the core layer 1012 excluding the portion that is to become the core is reduced.
Thus, claddings 1016 and a core 1013 are obtained. This method does not include a development process, that is, the method includes fewer steps and is performable at a low cost. Furthermore, the sidewalls of the core 1013 do not have surface roughness. Therefore, a polymer optical waveguide having a small propagation loss is realized. Nevertheless, since the refractive index in portions of the core layer 1012 on both sides of the portion that is to become the core is reduced by chemical reaction, it is difficult to increase the difference in refractive index in the horizontal direction, i.e., the lateral direction of the core 1013.
In addition, an optical waveguide as a stack of films is easy to bend in the stacking direction. Therefore, the difference in refractive index in the direction perpendicular to the holding surface is to be increased so that the loss due to such bending is reduced.
Consequently, the difference in refractive index between the core and the cladding differs between that in the vertical direction and that in the horizontal direction of the optical waveguide. Such a characteristic is referred to as “anisotropy” of the difference in refractive index.
The above technique is discussed in Japanese National Publication of International Patent Application No. 2008-535037 and by Naomi Kawakami et al. in R2010-6, CPM2010-6, and OPE2010-6 (2010-4), reports from the Institute of Electronics, Information and Communication Engineers.
If a polymer optical waveguide in which the difference in refractive index between the core and the cladding differs between that in the vertical direction and that in the horizontal direction with respect to the surface on which the waveguide is held is optically connected to an optical fiber in which the difference in refractive index is isotropic, some optical loss occurs and the optical performance is deteriorated as described below in the description of embodiments.
Similarly, if polymer optical waveguides having different numerical-aperture (NA) characteristics are optically connected to each other, some optical loss may occur.
Hence, the following embodiments provide a mechanism that reduces the optical connection loss occurring in a case where an optical waveguide having an anisotropic difference in refractive index and an optical waveguide having an isotropic difference in refractive index are connected to each other or in a case where optical waveguides having different NA characteristics and each have an anisotropic difference in refractive index are connected to each other.